A revised six-kingdom system of life

Evolutionary Biology Programme, Canadian Institute
for Advanced
Research, Department of Botany, University of British Columbia,
Vancouver, BC, Canada V6T 1Z4

Published online: 01 August 1998

Abstract

A revised six-kingdom system of life is presented, down to the
level of infraphylum. As in my 1983 system
Bacteria are treated as a single kingdom, and eukaryotes are divided into
only five kingdoms: Protozoa,
Animalia, Fungi, Plantae and Chromista. Intermediate high level categories
(superkingdom, subkingdom,
branch, infrakingdom, superphylum, subphylum and infraphylum) are extensively
used to avoid splitting
organisms into an excessive number of kingdoms and phyla (60 only being
recognized). The two ‘zoological’
kingdoms, Protozoa and Animalia, are subject to the International Code
of Zoological Nomenclature, the
kingdom Bacteria to the International Code of Bacteriological Nomenclature,
and the three ‘botanical’
kingdoms (Plantae, Fungi, Chromista) to the International Code of Botanical
Nomenclature. Circumscriptions
of the kingdoms Bacteria and Plantae remain unchanged since Cavalier-Smith
(1981). The kingdom
Fungi is expanded by adding Microsporidia, because of protein sequence
evidence that these amitochondrial
intracellular parasites are related to conventional Fungi, not Protozoa.
Fungi are subdivided into four phyla
and 20 classes; fungal classification at the rank of subclass and above
is comprehensively revised. The
kingdoms Protozoa and Animalia are modified in the light of molecular phylogenetic
evidence that Myxozoa
are actually Animalia, not Protozoa, and that mesozoans are related to
bilaterian animals. Animalia are
divided into four subkingdoms: Radiata (phyla Porifera, Cnidaria, Placozoa,
Ctenophora), Myxozoa,
Mesozoa and Bilateria (bilateral animals: all other phyla). Several new
higher level groupings are made in
the animal kingdom including three new phyla: Acanthognatha (rotifers,
acanthocephalans, gastrotrichs,
gnathostomulids), Brachiozoa (brachiopods and phoronids) and Lobopoda (onychophorans
and tardigrades),
so only 23 animal phyla are recognized. Archezoa, here restricted to the
phyla Metamonada and
Trichozoa, are treated as a subkingdom within Protozoa, as in my 1983 six-kingdom
system, not as a separate
kingdom. The recently revised phylum Rhizopoda is modified further by adding
more flagellates and
removing some ‘rhizopods’ and is therefore renamed Cercozoa.
The number of protozoan phyla is reduced
by grouping Mycetozoa and Archamoebae (both now infraphyla) as a new subphylum
Conosa within the
phylum Amoebozoa alongside the subphylum Lobosa, which now includes both
the traditional aerobic
lobosean amoebae and Multicilia. Haplosporidia and the (formerly
microsporidian) metchnikovellids are
now both placed within the phylum Sporozoa. These changes make a total
of only 13 currently recognized
protozoan phyla, which are grouped into two subkingdoms: Archezoa and Neozoa;
the latter is modified in
circumscription by adding the Discicristata, a new infrakingdom comprising
the phyla Percolozoa and
Euglenozoa). These changes are discussed in relation to the principles
of megasystematics, here defined as
systematics that concentrates on the higher levels of classes, phyla, and
kingdoms. These principles also make
it desirable to rank Archaebacteria as an infrakingdom of the kingdom Bacteria,
not as a separate kingdom.
Archaebacteria are grouped with the infrakingdom Posibacteria to form a
new subkingdom, Unibacteria,
comprising all bacteria bounded by a single membrane. The bacterial subkingdom
Negibacteria, with
separate cytoplasmic and outer membranes, is subdivided into two infrakingdoms:
Lipobacteria, which lack
lipopolysaccharide and have only phospholipids in the outer membrane, and
Glycobacteria, with
lipopolysaccharides in the outer leaflet of the outer membrane and phospholipids
in its inner leaflet. This
primary grouping of the 10 bacterial phyla into subkingdoms is based on
the number of cell-envelope
membranes, whilst their subdivision into infrakingdoms emphasises their
membrane chemistry; definition of
the negibacterial phyla, five at least partly photosynthetic, relies chiefly
on photosynthetic mechanism and
cell-envelope structure and chemistry corroborated by ribosomal RNA phylogeny.
The kingdoms Protozoa
and Chromista are slightly changed in circumscription by transferring subphylum
Opalinata (classes
Opalinea, Proteromonadea, Blastocystea cl. nov.) from Protozoa into infrakingdom
Heterokonta of the
kingdom Chromista. Opalinata are grouped with the subphylum Pseudofungi
and the zooflagellate
Developayella elegans (in a new subphylum Bigyromonada) to form
a new botanical phylum (Bigyra) of
heterotrophs with a double ciliary transitional helix, making it necessary
to abandon the phylum name
Opalozoa, which formerly included Opalinata. The loss of ciliary retronemes
in Opalinata is attributed to
their evolution of gut commensalism. The nature of the ancestral chromist
is discussed in the light of recent
phylogenetic evidence.